The invention relates to a method for filling a pressurized medium chamber of a level control system for a motor vehicle by which a vehicle body is suspended relative to at least one vehicle axle. The level control system includes: a pressurized medium store; at least one pressurized medium chamber which is operatively connected from time to time to the pressurized medium store so that pressurized medium from the pressurized medium store is transferrable into the pressurized medium chamber; and, a compressor which is operatively connected from time to time to the pressurized medium store or to the pressurized medium chamber in such a manner that pressurized medium is transferrable into the pressurized medium store or into the pressurized medium chamber by means of the compressor.
A level control system, which includes the pressurized medium chambers and a pressurized medium store, is known in the form of an air spring system as described, for example, in German patent publication 3,339,080. In this system, the pressurized medium can be transferred from the pressurized medium store into the pressurized medium chambers. When one of the pressurized medium chambers of the level control system is to be filled, then this pressurized medium chamber is connected to the pressurized medium store and, with the aid of a compressor, pressurized air is transferred from the pressurized medium store into the pressurized medium chamber. An advantage of the level control system disclosed in German patent publication 3,339,080 is that, because of the increased pressure in the pressurized medium store, the power consumed by the compressor is lower than when this pressurized air is transferred from the atmosphere into the pressurized medium chamber.
It can, however, be stated that the compressor runs during the entire filling operation of a pressurized medium chamber in the level control system disclosed in the German patent publication 3,339,080. This generates noise which is especially disturbing when the vehicle is at standstill and when there is an overall low noise level.
It is an object of the invention to provide a method for filling a pressurized medium chamber of a level control system for a motor vehicle wherein the pressurized medium chamber can be filled from a pressurized medium store as long as possible without actuating the compressor.
The method of the invention is for filling a pressurized medium chamber of a level control system for a motor vehicle by which a vehicle body is suspended relative to at least one axle of the vehicle, the level control system including: a pressurized medium store; at least one pressurized medium chamber which is operatively connected from time to time to the pressurized medium store so that pressurized medium from the pressurized medium store is transferrable into the pressurized medium chamber; and, a compressor which is operatively connected from time to time to the pressurized medium store or to the pressurized medium chamber in such a manner that pressurized medium is transferrable into the pressurized medium store or into the pressurized medium chamber by means of the compressor; the method includes the steps of: in advance of filling the pressurized medium chamber with the pressurized medium, determining the pressure in the pressurized medium store and the pressure in the pressurized medium chamber and the pressure difference between the pressurized medium store and the pressurized medium chamber; and, during the filling operation, transferring pressurized medium at least from the pressurized medium store to the pressurized medium chamber during a time span during which the pressure difference between the pressurized medium store and the pressurized medium chamber exceeds a threshold value; transferring pressurized medium into the pressurized medium chamber utilizing the compressor when the pressurized medium pressure difference between the pressurized medium store and the pressurized medium chamber drops below the threshold value and a further filling of the pressurized medium chamber is necessary.
For the threshold value, a pressure is preferably pregiven which is between 0 bar and 1 bar. The determination of the pressure in the pressurized medium store and in the pressurized medium chamber takes place preferably with the aid of a single pressure sensor of the pressure control system. How this is done will be explained in detail in connection with the drawings hereinafter.
The advantage achieved by the invention is especially seen in that a filling of a pressurized medium chamber of a level control system takes place essentially with the aid of the pressure store and the compressor is only utilized when the pressure difference between the pressurized medium store and the pressurized medium chamber reaches a threshold value. With the method of the invention, the time span is optimized during which the pressurized medium is transferred from the pressurized medium store into the pressurized medium chamber. With the invention, a substantially noiseless filling of the pressurized medium chamber from the pressurized medium store is therefore possible which is especially advantageous during standstill of the motor vehicle or when traveling at a low vehicle speed. The threshold value is preferably so pregiven that the pressure in the pressurized medium store is slightly greater than the pressure in the pressurized medium chamber when the threshold value is reached so that a sufficiently large air-mass flow of the pressurized medium is transferred from the pressurized medium store into the pressurized medium chamber when the threshold value is reached. A further advantage of the invention is that the level control system has a simple configuration and no additional costs are incurred compared to a conventional level control system.
According to another feature of the invention, the time span during which the pressure difference between the pressurized medium store and the pressurized medium chamber exceeds a threshold value is determined from a characteristic line which reflects the time-dependent course of the pressure difference. An advantage of this feature is that no pressure measurement is necessary during a filling operation in order to determine the actual pressure difference. A further advantage of this embodiment is that a determination of the time span from a characteristic line is possible in a simple manner. However, and alternatively, a computation of the time span with the aid of a mathematical model is possible.
According to another feature of the invention, the pressure in the pressurized medium store is measured directly after a change of the pressure in the pressurized medium store (that is, after each filling operation of the pressurized medium chamber by means of the pressurized medium store and after a filling operation of the pressurized medium store by means of the compressor). The measured pressure is stored in the control unit of the level control system and is available for the next filling operation to compute the starting pressure difference. The advantage of this embodiment is that only the pressure in the pressurized medium chamber has to be measured directly in advance of a filling operation and therefore no additional valve switchings for the stored pressure measurement are necessary directly in advance of a filling operation.
According to another feature of the invention, the pressure in the pressurized medium store is measured at regular intervals. Preferably, intervals for pressure measurement are selected which lie between 5 minutes and 20 minutes. The measured pressure is stored in the control unit of the level control system so that only the pressure in the pressurized medium chamber needs to be measured directly in advance of a filling operation. The advantage of this embodiment is that, with regular pressure measurements, pressure fluctuations in the pressurized medium store because of temperature fluctuations or leakage of the pressurized medium store are recognized and can be eliminated. Accordingly, an actual pressure of the pressurized medium store is always the basis in a filling operation.
According to another feature of the invention, the vehicle body is suspended relative to at least one vehicle axle by two pressure chambers and, for a common filling of the pressurized medium chambers, the pressure in each pressurized medium chamber is measured in advance of the fill operation and a mean value is determined from these measurement data. Pressurized medium is transferred over a time duration from the pressurized medium store into the pressurized medium chamber during which time duration the pressure difference between the pressurized medium store and the mean value exceeds a threshold value.
According to still another feature of the invention, the vehicle body is suspended relative to at least one vehicle axle by two pressure chambers and a mean value of the pressure in the pressurized medium chambers is measured in that the pressurized medium chambers per axle are connected via valves to a pressure sensor. The valves are assigned to corresponding ones of the pressurized medium chambers. The pressurized medium is transferred over a time duration from the pressurized medium store into the pressurized medium chambers and, during this time span, the pressure difference between the pressurized medium store and the mean value exceeds a threshold value. This feature of the invention has the advantage compared to the feature discussed immediately above that the mean value in the two pressurized medium chambers is determined directly. A separate connection of the individual pressurized medium chambers to the pressure sensor is not necessary.
According to still another feature of the invention, the pressure determination in one or more of the pressurized medium chambers takes place directly in advance of a filling operation. The advantage of this embodiment is that the filling operation is carried out on the basis of a precise pressure value which was present in the pressurized medium chambers directly in advance of the filling operation. In this way, a computation of the initial pressurized medium pressure difference and of the time duration during which this pressurized medium pressure difference is exceeded can take place precisely. A further advantage of the embodiment becomes understandable when one considers that, to determine the pressure or the pressure mean value, one or more of the pressure chambers are connected simultaneously to a pressure sensor by opening the valves assigned thereto. After determining the pressure or the pressure mean value, the above-mentioned valves remain open and a valve, which is assigned to the pressurized medium store, is additionally opened in order to connect this valve to the one or more pressurized medium chambers. According to this embodiment, the advantage is therefore achieved that no additional switching operations at the valves (which shorten their service life) are necessary notwithstanding the determination of the pressure or of the pressure mean value in one or more pressure chambers when the pressurized medium chambers are connected to the pressurized medium store for filling directly after the determination.
According to another feature of the invention, the compressor is not operatively connected to the pressurized medium store during the time span in which the pressurized medium is transferred into a pressurized medium chamber via the compressor. The advantage of this embodiment is that the compressor transfers pressurized medium only into the pressurized medium chambers during the filling operation and not additionally into the pressurized medium store. In this way, the filling operation of one or more pressurized medium chambers is not delayed by an unwanted filling of the pressurized medium store.
According to another embodiment of the invention, pressurized medium is transferred by means of the compressor into the pressurized medium chamber during the time span in which the pressurized means is transferred from the pressurized medium store into a pressurized medium chamber. The advantage of this embodiment is that a filling operation of one or more pressurized medium chambers can be completed especially rapidly. Preferably, use is made of this further embodiment when the speed of the vehicle exceeds a threshold value because, in this case, the noise generated by the running of the compressor is not disturbing.